Pressure sensitive adhesive (PSA) (also known as “self adhesive” or “self stick adhesive”) is adhesive which forms a bond at room temperature with a variety of dissimilar surfaces when light pressure is applied. No solvent, heat or radiation is needed to activate the adhesive. It finds wide applications in pressure-sensitive tapes, general purpose labels, post-it notes, postage stamps, and a wide variety of other products, e.g., packaging, automobile trim assembly, sound/vibration damping films, maternity and child care products like diapers, and hospital and first aid products like wound care dressings.
Nowadays, most commercially available PSAs are usually based on elastomeric materials compounded with optionally suitable tackifiers and waxes. The elastomeric materials are basically based on petrochemicals such as acrylics or modified acrylics, butyl rubber, silicone rubbers, ethylene-vinyl acetate copolymers, nitriles, vinyl ethers, and styrene block copolymers (e.g., SBS, SEBS, SEP and SIS). Tackifiers like C-5 and C-9 resins, and waxes, are usually low molecular weight polymers or oligomers which are also derived from petroleum streams. However, non-renewable petroleum is being depleted at a fast pace. On the other hand, many petrochemical-based polymers are not biodegradable, and their disposals after their useful life create serious environmental pollution. Therefore, it is desirable to provide PSA compositions that can be made from renewable natural materials; and it is highly desirable that the manufacture and use of PSA do not generate environmental pollution.
Plant oils are one of the most abundant and least expensive renewable raw materials, among which soybean oil is one of the most attractive. More than 20 billion pounds of soybean oils are produced annually in the United States, however, no higher than 600 million pounds of them are used for industrial application. Therefore, development of economically feasible new industrial products from plant oils is also highly desirable, especially from soybean oil which is facing a combination of a large annual surplus (with frequent carry-over exceeding 1 billion pounds) and the paucity of increased non-food uses for it. Plant oil is mainly a mixture of triglycerides with varying composition of long-chain saturated and unsaturated fatty acids depending on the plant, the crop, and the growing conditions. The double bonds in unsaturated fatty acids can be converted into more reactive oxirane moieties (epoxy functional groups) by appropriate reactions. Epoxidized plant oils such as epoxidized soybean oil (ESO) are commercially available and are well used in rubbers, plastics, resins, coatings, and various thermosetting composites.
The use of plant oils as starting materials for making PSAs has numerous advantages such as low cost, low toxicity, inherent biodegradability, and fairly high purity. Several general approaches for making PSAs from vegetable oils have been disclosed (see, e.g., WO 2008/144703). In the first approach, free-radically polymerizable functional groups such as acrylate or methacrylate groups are first introduced onto fatty acid, fatty esters or vegetable oils and then polymerized via a free radical polymerization method such as ultraviolet radiation to form PSAs. The introduction of the functional groups is typically accomplished through the reaction between epoxidized fatty esters or epoxidized oils and acrylic acid/methacrylic acid. During the polymerization, various acrylic or methacrylic monomers may be used to co-polymerize with acrylated fatty acids or esters or acrylated vegetable oils. In this approach, petrochemical-based acrylate is still used, and the procedure is complex and time-consuming. This approach is considered as an extension of traditional free radical polymerization methods of making petrochemical-based PSAs. In the second approach, fatty ester or vegetable oils are first epoxidized. The epoxidized fatty esters or epoxidized vegetable oils are then polymerized to form PSAs through cationically catalyzed ring-opening polymerization of the epoxy rings. Some other epoxidized compounds may be copolymerized with epoxidized fatty esters or epoxidized vegetable oils for improving the properties of PSAs. The third approach involves the direct polymerization of carbon-carbon double bonds on fatty acids, fatty esters or vegetable oils with other free-radically polymerizable compounds such as acrylate or methacrylate. Unlike those in drying oils such as Tung oil, most carbon-carbon double bonds in vegetable oils are not conjugated, thus having relatively low reactivity during the free radical polymerization. In this third approach, fatty acids, fatty esters or vegetable oils have to be modified to form conjugated double bonds before the free radical polymerization.